1. Field of the Invention
The present invention relates to a method of manufacturing an optical element. In particular, the invention relates to a method of manufacturing an optical element having an aspherical shape.
2. Brief Description of Related Art
The optical element having the optical surface is, for example, an optical component such as an optical lens or an optical mirror used in optical systems, such as telescopes used in astronomy, and systems used for imaging structures, such as structures formed on a mask or reticle, onto a radiation sensitive substrate, such as a resist, in a lithographic method. The success of such an optical system is substantially determined by the accuracy with which the optical surface can be processed or manufactured to have a target shape determined by a designer of the optical system. In such manufacture it is necessary to compare the shape of the processed optical surface with its target shape, and to determine differences between the processed and target surfaces. The optical surface may then be further processed at those portions where differences between the processed and target surfaces exceed e.g. predefined thresholds.
Interferometric apparatuses are commonly used for high precision measurements of optical surfaces. Examples of such apparatus are disclosed in U.S. Pat. No. 4,732,483, U.S. Pat. No. 4,340,306, U.S. Pat. No. 5,473,434, U.S. Pat. No. 5,777,741, U.S. Pat. No. 5,488,477. The entire contents of these documents are incorporated herein by reference.
The conventional interferometer apparatus for measuring a spherical optical surface typically includes a source of coherent light and an interferometer optics for generating a beam of measuring light incident on the surface to be tested, such that wavefronts of the measuring light have, at a position of the surface to be tested, a same shape as the target shape of the surface under test. In such a situation, the beam of measuring light is orthogonally incident on the surface under test, and is reflected therefrom to travel back towards the interferometer optics. Thereafter, the light of the measuring beam reflected from the surface under test is superimposed with light reflected from a reference surface and deviations of the shape of the surface under test and its target shape are determined from a resulting interference pattern.
While spherical wavefronts for testing spherical optical surfaces may be generated with a relatively high precision by conventional interferometer optics, more advanced optics, which are also referred to as compensators, null lens arrangements, or K-systems, are necessary to generate beams of measuring light having aspherical wavefronts such that the light is orthogonally incident at each location of the aspherical surface under test. Background information relating to null lens arrangements or compensators is available e.g. from the text book of Daniel Malacara “Optical Shop Testing”, 2nd Edition, John Wiley & Sons, Inc. 1992, Chapter 12.
The compensator for generating the aspherical wavefronts may comprise one of more refractive optical elements, such as lenses. It is also known to use a diffractive element such as a hologram in a compensator for generating the aspherical wavefronts. Background information and examples of using holograms in interferometric measurements are illustrated in Chapters 15.1, 15.2, and 15.3 of the text book of Daniel Malacara mentioned above. The hologram may be a real hologram generated by exposing a suitable material, such as a photographic plate, with interfering light beams, or a synthetic hologram, such as a computer generated hologram (CGH) generated by simulating the interferometer set up by a suitable computational method, such as ray tracing, and producing the hologram by manufacturing steps using a pen plotter and optical reduction, lithographic steps, laser beam recorders, electron beam recorders and others.
It has been found that the conventional methods of testing and manufacturing aspherical optical surfaces using a hologram have an insufficient accuracy in some applications.